The former US President Bush stated in 2003 that “with a new national commitment, our scientists and engineers will overcome obstacles to taking these cars from laboratory to showroom, so that the first car driven by a child born today could be powered by hydrogen and pollution free”. Not only were the American scientists inspired by President Bush's speech, many other scientists of the whole world also devoted themselves to this research. With hydrogen now being a viable source to power vehicles, considerable research has been performed on designing internal combustion engines to run on hydrogen rather than fossil fuels. In these designs, a hydrogen/air mixture is combusted inside an internal combustion engine much like gasoline and other hydrocarbon fuels are combusted in present day internal combustion engines. With hydrogen, however, catalytic converters are not needed to treat the exhaust to comply with emission standards. Hydrogen burns clean with the only byproduct being water.
On the other hand, considerable research has also been performed on using fuel cells to power vehicles. Fuel cells, like batteries, operate by utilizing electrochemical reactions. Unlike a battery, in which chemical energy is stored within the cell, fuel cells generally are supplied with reactants from outside the cell to produce an electrical current used to power a vehicle. In fuel cells, a hydrogen stream, an oxygen stream, and an electrolyte stream are used to provide an electric current. Fuel cells offer a number of important advantages over internal combustion engine or generator systems. These include relatively high efficiency, environmentally clean operation, high reliability, few moving parts, and quiet operation. Fuel cells potentially are more efficient than other conventional power sources based upon the Carnot cycle.
Among many types of fuel cells, PEM fuel cell has drawn the most attention because of its simplicity, viability, quick start-up, and it has been demonstrated in almost any conceivable application, from powering a cell phone to a locomotive. PEM stands for Polymer Electrolyte Membrane or Proton Exchange Membrane. In early 1960's, these cells were known as Solid Polymer Electrolyte fuel cells as an auxiliary power source in the Gemini space flights.
While hydrogen has wide potential application as a fuel, a major drawback in its utilization, especially in limited space or mobile uses such as the powering of vehicles, has been the lack of acceptable lightweight compact hydrogen storage medium. Conventionally, hydrogen has been stored in pressure-resistant vessels under a high pressure or stored as a cryogenic liquid, being cooled to an extremely low temperature. Storage of hydrogen as a compressed gas of liquid involves the use of large and/or cryogenic vessels, making the use of hydrogen to power vehicles less feasible.
Fortunately, storing hydrogen in its affinitive alloys (also named hydrogen storage alloys) is safe and efficient when compared to the method of pressurizing hydrogen gas into high-strength alloy container. The same amount of hydrogen when absorbed in alloy occupies only one thousandth of the volume compared to when it is in a gas state at one atmosphere. Regarding safety, if hydrogen leaks from the hydrogen storage alloy accidentally, further release rates will decrease as a result of the drop of temperature. So, hydrogen storage alloys have long been commercially used in hybrid cars and the electrode of Ni—H batteries for 3C electronic products. The more recent appeal of these alloys is in their potential application in fuel cell energy. The use of a hydrogen storage alloy is one of the ideal methods for hydrogen storage and transportation, because the volume density of hydrogen is high or even higher than that of liquid or solid hydrogen.